Glucose, sucrose, starch and cellulose are the most abundant renewable sources found in the earth for carbohydrates. These carbohydrates have high contents of oxidation groups, so it is very favorable to manufacture basic chemicals with them. Especially, without releasing carbon dioxide, these carbohydrates are the most attractive raw materials for synthesis of chemical intermediates in a sustainable manner.
Theoretically, each mole of hexose could be converted into two moles of lactic acid through fermentation or catalytic reaction. Lactic acid is a monomer for synthesis of biodegradable polylactic acids. Lactic acid and derivatives thereof (such as alkyl lactates and polylactic acids) may be used for synthesis of other three-carbon compounds such as propylene glycol, acrylic acid and allyl alcohol, and these three-carbon compounds may be used for synthesis of other polymers.
In the current chemical industry, lactic acid is produced by fermentation of glucose, referring to FIG. 1. However, the fermentation process only provides a very dilute lactic acid fermentation broth (<10% aqueous solution) which may react with calcium hydroxide to afford solid calcium lactate which then reacts with a sulfuric acid solution to afford lactic acid. The fermentation process will produce massive wastewater and solid calcium sulfate waste, and the fermentation process of lactic acid only uses glucose as the raw material. Existing fermentation processes can produce lactic acid in a large scale (120,000 tons/year) with glucose. However, the problem with microbial fermentation is that the reaction rate is low and the concentration of the product is low (in water), so the reaction time is long, reactors with large volumes are required, and the energy consumption during product purification is high (please refer to Fermentation of Glucose to Lactic Acid Coupled with Reactive Extraction: Kailas L. Wasewar, Archis A. Yawalkar, Jacob A. Moulijn and Vishwas G. Pangarkar, Ind. Eng. Chem. Res. 2004, 43, 5969-5982).
As is well known in the art, in the presence of alkali metal hydroxides, monosaccharides could be converted into lactic acid (please refer to R. Montgomery. Ind. Eng. Chem, 1953, 45, 1144; B. Y. Yang and R. Montgomery, Carbohydr. Res. 1996, 280, 47). However, stoichiometric alkali (Ca(OH)2) and acid (H2SO4) will be consumed in the recovery process of lactic acid. Thus, corresponding stoichiometric salt waste is produced.
The commercial fermentation process can produce massive lactic acid, but it only uses starch as the raw material, and starch must be pre-hydrolyzed (or fermented) so as to afford glucose. The fermentation process produces massive wastewater and solid waste (CaSO4). Moreover, the fermentation process for producing lactic acid includes many steps and requires consuming considerable energy. The essential equipments required by the fermentation process are very complicated and not economical.